1. Field of the Invention
The present invention relates to filter cakes of the type formed during well bore operations and particularly to a method for increasing the permeability of the formation through the use of an enzyme treatment to break down a polysaccharide-containing filter cake.
2 Description of the Prior Art
Filter cakes or face plugs form during various procedures done within a well bore. Filter cakes are composed of precipitates, such as silicates formed from drilling muds, or residue formed after using polymer-containing gelatable fluids. The residue can contain either polyacrylamide or polysaccharides, depending on the polymer used. The method of the invention relates to polysaccharide residues, particularly filter cakes.
During hydraulic fracturing, one type of well bore procedure, a sand laden fluid is injected into a well bore under high pressure. Once the natural reservoir pressures are exceeded, the fracturing fluid initiates a fracture in the formation which generally continues to grow during pumping. The treatment design generally requires the fluid to reach maximum viscosity as it enters the fracture which affects the fracture length and width. This viscosity is normally obtained by the gelation of suitable polymers, such as a suitable polysaccharide, and are known as fracturing gels. The gelled fluid can be accompanied by a propping agent which results in the placement of the propping agent within the fracture thus produced. The proppant remains in the produced fracture to prevent the complete closure of the fracture and to form a conductive channel extending from the well bore into the formation being treated once the fracturing fluid is recovered. Propping agents include a wide variety of material and may be coated with resins. The gel fluids may also contain other conventional additives common to the well service industry such as surfactants, and the like.
Occasionally, production from well bore operations must cease temporarily to perform auxiliary procedures called workover operations. The use of temporary blocking gels, also formed by gelation of appropriate polysaccharides, produces a relatively impermeable barrier across the production formation.
At the end of fracturing or workover operations the gels are degraded and the fluids are recovered. The recovery of fracturing and blocking gel fluids is accomplished by reducing the viscosity of the fluid to a low value such that it flows naturally from the formation under the influence of formation fluids and pressure. This viscosity reduction or conversion is referred to as "breaking."
Polysaccharides have other uses within the oil industry. For example, polysaccharides thicken fluids and control fluid loss. Other types of polysaccharides are used with proppants, such as sand control fluids and completion fluids
Filter cakes, however, often form during these procedures. A filter cake is a tough, dense, practically water insoluble residue that reduces the permeability of the formation. The concentration of polysaccharide within a filter cake is greater than the normal polysaccharide concentration in a fracturing fluid, for instance, 480 pounds per thousand (ppt) versus 40 ppt. See, S.P.E. Publication No. 21497.
Filter cakes form in a variety of ways. For example, when the gel fluids are pumped into the subterranean formation, fluid may leak into the formation matrix through the pore spaces of the rock. The pores act as filters, permitting the fluid to leak into the rock matrix while filtering out the gel. A layer of the filtered gel deposits on the face of the matrix and plugs the formation. Incomplete gel degradation is another example.
Filter cake interferes with production from the formation by decreasing the output of hydrocarbons. Filter cake fills the rock matrix pores and curtails the flow of fluids from the matrix. When a fracture closes at the end of treatment, the closure may force the remaining filter cake into the proppant bed and nearby flow channels. The filter cake can then plug the flow channels, thereby reducing the flow of hydrocarbons during production.
Although some polysaccharides do not form filter cakes, the viscosity of these fluids creates damaging conditions analogous to those found with filter cakes. Therefore, the term "filter cake" when used as a generic term in this disclosure may also refer to these conditions.
The permeability of the formation may be assessed in the laboratory. One procedure of assessing the permeability measures the flow of a fluid through a damaged formation sample at a given rate and pressure. For example, a completely broken filter cake regains greater than about 95% of the initial permeability of a formation sample using a damage permeability test, while a plugged formation has about 30% of the initial permeability, depending on the fluid, core and conditions. A second procedure assesses the retained conductivity of the formation. A plugged formation has a retained conductivity of less than 10%, depending on the conditions.
Therefore, removal of filter cake is necessary to increase the flow of production fluids from the formation. Since filter cake is dense and practically insoluble in aqueous fluids, it cannot be merely flushed out of the formation. Removal of filter cake requires some additional treatment. Common oxidants, for example persulfates, are often used to remove filter cake. The oxidants, however, are ineffective at low temperature ranges from ambient temperature to 130.degree. F. In this temperature range the oxidants are stable and do not readily undergo homolytic cleavage to initiate the degradation of the filter cake. Cleavage is typically achieved at lower temperatures only by using high concentrations of oxidizers. High oxidizer concentrations are frequently poorly soluble under the treatment conditions.
Reactions with common oxidants, however, are difficult to control. Common oxidants break polysaccharides into nonspecific units, creating a filter cake consisting of a mixture of monosaccharide, disaccharide and polysaccharide fragments as well as other miscellaneous fragments. Common oxidants react with things other than the gel fragments. Oxidants can react with iron found in the formation, producing iron oxides which precipitate and damage the formation, thereby decreasing permeability. Oxidants can also react nonspecifically with other materials used in the oil industry, for example, tubings, linings and resin coated proppants.
Oxidants can break down any subsequent gels used in the formation. If the oxidants are not completely removed or inactivated, they can prematurely break the new gel. Therefore, oxidants must be completely removed or inactivated before any subsequent introduction of another gel into the subterranean formation.
To completely remove the filter cake after treating with oxidants, additional treatment may be required. An extra acid hydrolysis step may be necessary to remove residual residue. Treatment with an acid for example, hydrochloric acid, augments the removal of excess residue. Acid treatments, however, corrode steel and equipment used in the operation. Acid treatments may also be incompatible with the formation and/or its fluids.
Fluoride ions paired with an oxidant in an acidic environment increases the efficiency of filter cake removal. Free fluoride ions compete with the polysaccharide polymer for the metal ions of the crosslinker. The crosslinker metal ions have a greater affinity for fluoride than for the polysaccharide in the gel. This affinity paired with the action of an oxidizer breaks the gel more quickly.
One of the problems associated with fluoride is that the free ions are extremely reactive. Unfortunately, fluoride can react with most metals and many nonmetals. Free fluoride ions can react with the metals in the tubing and the formation. For example, fluoride ions readily react with calcium, forming calcium fluoride which precipitates in aqueous solutions and damages the production zone.
To circumvent the problem of the reactivity of fluoride ions, the prior art suggests the addition of boron which has a high affinity for fluoride. Yet, no amount of added boron can counteract the amount of calcium present in many formations, such as limestone which is calcium carbonate. Calcium carbonate is insoluble in water and soluble in acid solutions whereas calcium fluoride is only slightly soluble in water and acid solutions. These conditions favor the formation of calcium fluoride. To prevent the formation of calcium fluoride in a limestone formation, at a minimum the boron would have to be in excess of the fluoride, hence interfering with the efficiency of the reaction by reducing the amount of fluoride available to react with the crosslinker of the filter cake.
Enzyme systems are known to degrade the types of polysaccharides used in fracturing, blocking gels and other oil industry applications. Enzyme breaker systems have been designed to break gelled fracturing and blocking fluids used in the industry. See, for example, the pending applications of Robert Tjon-Joe-Pin entitled "Enzyme Breaker For Galactomann, an Based Fracturing Fluid", U.S. Pat. No. 5,201,370, and Robert Tjon-Joe-Pin, et al., "Novel Enzyme Complex Used For Breaking Crosslinked Cellulose Based Blocking Gels At Low To Moderate Temperatures", U.S. Pat. No. 5,224,544. Enzymes, for example the cellulases, hemi-cellulases, amylases, pectinases, and their mixtures are familiar to those in the well service industry when used in fracturing gel breaker systems. Some of these enzymes break the bonds that connect the monosaccharides into a polysaccharide backbone, for instance, the (1,4)-.alpha.-D-galactosiduronic linkages in pectin. These conventional enzymes are nonspecific mixtures that cause random breaks. Therefore using these conventional enzymes to break gelled fracturing fluids results in only a partial degradation of the polysaccharide polymer. Instead of fragmenting almost completely into much smaller fragments, these enzymes break the polysaccharide backbone into a mixture of fragments consisting of monosaccharides, disaccharides and polysaccharides. Larger crosslinked fragments like disaccharides and polysaccharides can form filter cakes and damage the production zone. Since the breaks are nonspecific, conventional enzymes can degrade other components used in the system.
The present invention has as its object to provide a mechanism for degrading a filter cake formed during fracturing and other well bore operations which produces a rapid degradation of the filter cake to allow increased permeability of the formation and enhanced recovery of the formation fluids.
Another object of the invention is to provide a system for degrading a filter cake at low to moderate temperatures.
Another object of the invention is to provide an enzyme system that degrades the crosslinked residue of the filter cake primarily into monosaccharide fragments.
Another object of the invention is to provide an enzyme system that reduces the viscosity of noncrosslinked polysaccharides by degrading the polysaccharides into smaller pieces.
Another object of the invention is to provide a mechanism for degrading filter cake that does not react with other materials or metals used in well bore operations or found within the subterranean formation.